1. Field of the Invention
The present invention relates to a display panel, and in particular, to a manufacturing method of a display device using an insulating substrate with active elements formed in a strip-shaped poly-crystalline semiconductor film, obtained by reforming an amorphous or granular poly-crystalline semiconductor film formed on the main surface of the insulating substrate so as to expand crystal grains into a substantially strip shape by use of annealing with laser light (also referred to merely as laser hereinafter) irradiated thereto. The case where silicon is used as a semiconductor will be described below.
2. Description of the Related Art
In current display devices such as liquid crystal and organic electroluminescence display units, an image is formed by switching pixel transistors (thin film transistors: TFT) consisting of an amorphous or poly-crystalline silicon film on a substrate of glass, fused quartz or the like with an insulating film interposed therebetween. The substrate on which pixel circuits are formed is referred to as an active matrix substrate, TFT substrate, or is simply referred to as an active panel. If it becomes possible to form the driver circuit for driving the thin-film transistors of the pixel circuits concurrently with the formation of the thin-film transistors, drastic reduction in production cost and enhancement in reliability can be expected. However, when an amorphous silicon is used as a silicon semiconductor film constituting active layers of the transistors (thin film transistors) constituting the driver circuit, the performance of the thin film transistors, typically including mobility, is low. Thus, it is difficult to manufacture a circuit for which high speed and high function are required, from an amorphous silicon.
In order to manufacture such high-speed and high-function circuits, a high-mobility thin film transistor is required. To realize this, it is necessary to improve the crystallinity of the silicon thin film. As a method for improving the crystallinity, excimer laser annealing has been getting a lot of attention. According to this method, mobility is improved by irradiating an amorphous silicon film formed on an insulating substrate of glass or the like with an insulating film interposed therebetween with excimer laser so that the amorphous silicon thin film is transformed into a granular poly-crystalline silicon thin film. However, in the granular poly-crystalline silicon thin film obtained by irradiation of excimer laser, the particle size is about several tens to hundreds of nanometers. Thus, such a film is deficient in performance to be applied to a driver circuit or the like for driving pixel transistors in a display device.
As the prior art for overcoming this problem, “patent document 1” discloses a method of forming so-called strip-shaped crystals by linearly condensing temporally modulated continuous-wave (CW) laser light or quasi-continuous-wave laser light and by scanning and irradiating at a high speed in a direction which crosses (normally the direction which perpendicularly crosses) the longitudinal direction of the linear shape in the scanning direction to cause growth of crystals in the lateral direction (that is, scanning direction). According to this method, the entire surface of the substrate is poly-crystallized by the excimer laser annealing, and then only a region where driver circuits are formed is scanned by a laser beam in the direction which coincides with the current path (source-drain) of the formed transistors so that crystal grains are grown laterally. As a result, the mobility is greatly improved by the absence of crystal grain boundaries which traverse the current path or by reforming the crystal grains into minute crystals.
However, while high-output continuous-wave laser light or quasi-continuous-wave laser light is condensed by a transmission lens and irradiated, components of an objective lens absorb laser light, though to a small extent, and thus its temperature is increased. This temperature rise causes thermal expansion and a change in the index of refraction, and therefore the focal length of the lens is changed and the light-concentration ability of the lens is lowered. This change in the focal length or lowered light-concentration ability lowers the energy density on the surface of the substrate, resulting in a situation where energy for forming a strip-shaped poly-crystalline thin film is insufficient. Consequently, the quality and yield of the flat display device are lowered. Patent document 2 and patent document 3, among others, are known as methods for dealing with such changes in the focal position caused by this thermal lens effect.
Patent document 2 discloses a laser processing apparatus in which a spacer constituted of an expansion component which expand and shrink in response to a conductive heat from a condenser lens is provided in an upper part of a condenser lens, and the condenser lens is pressed downwardly towards a workpiece (the component to be processed) by the expansion component in response to its conductive heat in a state that the condenser lens is thrusted from the bottom to the top with a ring having a spring characteristic, so that the focal position is maintained at an optimum value by offsetting by the upward shift of the focal position by the thermal lens effect.
Patent document 3 discloses an apparatus in which a temperature in each point of a condenser lens is measured by a far-infrared radiant temperature sensor; the measurement is input into a computer having thermal analyzer software determined in advance to calculate the thermal deformation amount of the lens; the amount of a change in the focal length is calculated from the thermal deformation amount; and the position of the lens is moved depending on the results by using a pulse motor to control in such a manner that a focus is always on a workpiece material.
[Patent document 1]
Japanese Unexamined Patent Application Publication No. 2003-124136
[Patent document 2]
Japanese Unexamined Patent Application Publication No. H10-258381
[Patent document 3]
Japanese Unexamined Patent Application Publication No. H2-6093